1. The Field of the Invention
The present invention relates generally to upstream data communications over networks primarily designed for downstream transmission of television and data signals, and particularly to a system and method for converting, transmitting, and regenerating one or more data signals based on a single master clock.
2. Background and Relevant Art
Cable television systems (CATV) were initially deployed so that remotely located communities were allowed to place a receiver on a hilltop and then use coaxial cable and amplifiers to distribute received signals down to the town which otherwise had poor signal reception. These early systems brought the signal down from the antennas to a “head end” and then distributed the signals out from this point. Since the purpose was to distribute television channels throughout a community, the systems were designed to be one-way and did not have the capability to take information back from subscribers to the head end.
Over time, it was realized that the basic system infrastructure could be made to operate two-way with the addition of some new components. Two-way CATV was used for many years to carry back some locally generated video programming to the head end where it could be up-converted to a carrier frequency compatible with the normal television channels.
Definitions for CATV systems today call the normal broadcast direction from the head end to the subscribers the “forward path” and the direction from the subscribers back to the head end the “return path”. A good review of much of today's existing return path technology is contained in the book entitled Return Systems for Hybrid Fiber Coax Cable TV Networks by Donald Raskin and Dean Stoneback, hereby incorporated by reference as background information.
One additional innovation has become pervasive throughout the CATV industry over the past 10 years—the introduction of analog optical fiber transmitters and receivers operating over single mode optical fiber. These optical links have been used to break up the original tree and branch architecture of most CATV systems and to replace that with an architecture labeled Hybrid Fiber/Coax (HFC). In this approach, optical fibers connect the head end of the system to neighborhood nodes, and then coaxial cable is used to distribute signals from the neighborhood nodes to homes, businesses and the like in a small geographical area. Return path optical fibers are typically located in the same cable as the forward path optical fibers so that return signals can have the same advantages as the forward path.
FIG. 1 is a block diagram of a prior art cable television system 100 that uses conventional analog return path optical fiber links. Each subtree 102 on the system comprises a coaxial cable 106 that is coupled to a cable modem 108, each cable modem 108 being used by subscribers for Internet access. The coaxial cable 106 is also coupled to set top boxes (not shown) and other equipment (not shown), which are not relevant to the present discussion. The coaxial cable 106 of each subtree 102 is further coupled to at least one forward path optical fiber 110 and at least one return path optical fiber 112, typically through a cable node. An analog optoelectronic transceiver 114 (typically at the cable node) provides the data path coupling the coaxial cable 106 to the optical fibers 110, 112.
An RF input signal, having an associated signal level, is submitted to a′, transmitter portion of the optoelectronic transceiver 114, which in turn gains or attenuates the signal level, as appropriate. The RF input signal is then amplitude-modulated, and converted into a corresponding optical signal by a laser diode 122. Both Fabre-Perot (FP) and distributed feedback (DFB) lasers are typically used for this application. DFB lasers are used in conjunction with an optical isolator, and have improved signal to noise over FP lasers, but at a sacrifice of substantial cost. DFB lasers are preferred, as the improved SNR allows for better system performance when aggregating multiple returns.
The optical signal from the laser diode 122 is coupled to a single mode optical fiber (i.e., the return path optical fiber 112) that carries the signal to an optical receiver 130 typically located at a cable hub 350 (see FIGS. 7 and 8), such as a cable hub 350 at the head end system 132. The optical receiver 130 converts the amplitude-modulated light signal back to an RF signal. Sometimes a manual output amplitude adjustment mechanism is provided to adjust the signal level of the output produced by the optical receiver. A cable modem termination system (CMTS) 134 at the head end 132 receives and demodulates the recovered RF signals so as to recover the return path data signals sent by the subscribers.
FIGS. 2 and 3 depict the transmitter 150 and receiver 170 of a prior art return path link. The transmitter 150 (e.g., a cable node) digitizes the RF signal received from the coaxial cable 106, using an analog to digital converter (ADC) 152. The ADC 152 generates a ten-bit sample value for each cycle of the receiver's sample clock 153A, which is generated by a local, low noise clock generator 156. The output from the ADC 152 is converted by a serializer 154 into a serial data stream. The serializer 154 encodes the data using a standard 8B/10B mapping (i.e., a bit-value-balancing mapping), which increases the amount of data to be transmitted by twenty-five percent. This encoding is not tied to the 10-bit boundaries of the sample values, but rather is tied to the boundary of each set of eight samples (80 bits), which are encoded using 100 bits.
When the sample clock operates at a rate of 100 MHz, the output section of the serializer 154 is driven by a 125 MHz clock 157A, and outputs data bits to a fiber optic transmitter 158, 159 at a rate of 1.25 Gb/s. The fiber optic transmitter 158, 159 converts electrical 1 and 0 bits into optical 1 and 0 bits, which are then transmitted over an optical fiber 112. The fiber optic transmitter includes a laser diode driver 158 and a laser diode 159.
The receiver 170 at the receive end of the optical fiber 112 (e.g., a cable hub) includes a fiber receiver 172, 174 that receives the optical 1 and 0 bits transmitted over the optical fiber 112, and converts them into corresponding electrical 1 and 0 bits. This serial bit stream is conveyed to a deserializer circuit 178. A clock recovery circuit 176 recovers a 1.25 GHz bit clock from the incoming data and also generates a 100 MHz clock that is synchronized with the recovered 1.25 GHz bit clock.
The recovered 1.25 GHz bit clock is used by the deserializer 178 to clock in the received data, and the 100 MHz clock is used to drive a digital to analog converter 180, which converts ten-bit data values into analog voltage signals at the head end system. As a result, the RF signal from the coaxial cable 106 is regenerated at point 182 of the head end system.
Prior art return path link systems, such as the one shown in FIGS. 2 and 3, have used a low noise oscillator at the transmitter for the A/D sample clock 152. The same oscillator is also used as a reference for a synthesizer that generates a coherent symbol clock 157A for the communications link. The receiver 170 recovers the symbol clock 157B (e.g., via clock recovery circuit 176).
Time jitter is introduced in the receiver sample clock (e.g., via circuit 176) by the described communications path. The receiver's clock recovery circuit must react quickly to maintain lock on the received data. Accordingly, an advantage in the art can be realized with systems that maintain a consistent, approximate in-frequency replicate of a master clock rate in the return paths. Furthermore, in some cases, it may be desirable to transmit data other than television programming in a forward path as well. Such forward path data must also be properly synchronized. As there can be jitter introduced in both the return and forward paths, it would be advantageous to have a clocking mechanism that was reliable in both the return and forward data paths of the CATV network. It would further be desirable if the clock mechanism was of low complexity.